By way of background, compression molding is a technique often used to make fiber reinforced plastic (FRP) parts. A charge containing a curable resin is placed between upper and lower heated die members defining a mold cavity. The dies are then brought to a closed position where the dies compress the charge causing it to flow and fill the mold cavity. After the resin cures, the molds are opened and the finished part removed.
Compression molding techniques have been used to make parts having relatively flat surfaces, such as exterior automotive body panels. The charges used for making such parts generally consist of a thermosetting resin containing reinforcing fibers and various fillers. The charges are often formed into sheets known in the art as sheet molding compound (SMC). Unfortunately, problems have been experienced in the past in consistently providing molded parts with extremely smooth surface finishes as is demanded by the automotive industry. It is believed that an uneven filling rate of the charge within the mold can cause air to be entrapped within the molded part thereby resulting in substantial porosity. This uneven filling can also cause insufficient dispersion of the constituents within the mold. The porosity and non-uniform distribution, particularly of reinforcement fibers, can cause surface problems. Air entrapment between layers can also create blistering of the part resulting in a lack of durability.
According to the discussion in the 1974 U.S. Pat. No. 3,840,239 to Fazekas et al there have been a variety of methods that have been tried in order to reduce surface defects. Among them include: a) experimenting with different ranges of plasticity; b) increasing the molding pressure; c) lowering the temperature of the mold; d) closing the mold faster and applying a high pressure sooner; e) preheating the charges of molding compound; f) experimenting with different charge weights; g) trying a higher density molding compound to obtain greater "back pressure" in the mold; h) breathe the mold earlier; and i) use the semi-positive molds. The patentee in the '239 patents reports that none of the above methods have provided any real success in the attempt to eliminate porosity.
Another method used to avoid entrapped air was to use a relatively thick charge that covers a relatively small area of the molding surface. The idea was to attempt to "squeeze" the air out of the charge by slowly closing the molds. Preheating the charge tended to be counterproductive because it was often necessary to lower the temperature of the heated dies since the charge would otherwise gel before it could flow and fill the mold. Consequently, preheating SMC charges did not appear to provide any substantial benefits.
In recent years, it has been common to use what is known as an "in-mold coating" technique to provide fiber reinforced plastic (FRP) parts with a commercially acceptable smooth finish. This "in-mold coating" technique is disclosed, for example, in U.S. Pat. No. 4,081,578 to van Essen et al. Briefly, this method employs an additional processing step where the cured part remains in the mold and is coated with a composition that spreads and penetrates the surface to fill the pores and voids therein. Unfortunately, this technique has several drawbacks. For example, the additional coating operation consumes valuable machine time and lessens the amount of production which can be gained from a single mold. Relatively sophisticated and expensive mechanisms must be utilized to control the application of the coating to the part surface, and care must be taken to ensure that the coating will properly bond to the surface of the part.
The use of vacuum during the compression molding process has been recorded in the literature such as the '239 patent to Fazekas referenced above and the article by Gorsuch et al, entitled "Surface Porosity and Smoothness of SMC Molding as Affected by Vacuum and Other Molding Variables", 33rd Annual Technical Conference, 1978, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Section 9-F, pages 1-7. More recently, the assignee of the present invention has utilized vacuum in successfully molding acceptable parts without the need for the "in-mold coating" technique. See, for example, U.S. Pat. Nos. 4,488,862; 4,551,085 and U.S. patent application Ser. No. 640,470 entitled "Compression Molding a Charge Using Vacuum" by Iseler et al, now U.S. Pat. No. 4,612,149.